Sorbent based blood dialysis systems operate by circulating blood and dialysate fluid through a dialyzer on opposite sides of a membrane within the dialyzer. Blood waste compounds can move across the membrane from the blood side to the dialysate side. The dialysate fluid containing blood waste compounds can then be recycled by removing or breaking down the blood waste compounds. The process for removing urea, for example, can involve exposing the dialysate fluid to a material that breaks down urea into ammonium ions and carbonate. The byproduct ammonium ions are then removed by binding to an ion exchange material such as zirconium phosphate. Even though the ammonium ions are safely and easily removed by this process, it is sometimes desirable to monitor the presence of ammonium ions in dialysate fluid.